Eolian erosion of polygons in the Atacama Desert as a proxy for hyper-arid environments on Earth and beyond

Polygonal networks occur on various terrestrial and extraterrestrial surfaces holding valuable information on the pedological and climatological conditions under which they develop. However, unlike periglacial polygons that are commonly used as an environmental proxy, the information that polygons in the hyper-arid Atacama Desert can provide is little understood. To promote their use as a proxy, we investigated a polygonal network within an inactive channel that exhibits uncommonly diverse surface morphologies and mineral compositions, using geochemical and remote sensing techniques. Our findings show that the polygons belong to a continuous network of the same genetic origin. Their differences result from post-formational differential eolian erosion up to 50 cm depth, exposing indurated subsurface horizons rich in sulfate or nitrate and chloride. Their location in an ancient channel could lead to the misinterpretation of fluvial polygon erosion, however, we find no such signs but evidence for aqueous resurfacing of microtopography by fog and minimal rainwater infiltration. Our findings extend the use of polygons as proxies in the Atacama Desert, indicating saline soils and hyper-arid conditions. We conclude that this example of polygon erosion can guide future polygon research, especially regarding the use of erosional surfaces on Earth and beyond to gain valuable subsurface insights.


Scientific Reports
| (2022) 12:12394 | https://doi.org/10.1038/s41598-022-16404-y www.nature.com/scientificreports/ use as a proxy is likely to be different as well. PNs in the Atacama Desert exhibit significant differences, especially because of their soil cementation by salt instead of ice, the lack of enduring freezing temperatures, and more restricted availability of water. Although many studies investigated the climatological and pedological evolution of the Atacama Desert in the last decades [12][13][14][15][16][17][18] , the use of PNs as an environmental proxy for climate and soil type has not been realized in this context. The Atacama Desert is particularly well suited for studying the use of PNs as a proxy for long-term environmental conditions because it represents the oldest non-polar desert and exhibits hyper-aridity over the last ~ 12 Ma 19 , intercepted by stages of semi-arid to arid climate 18 . The low precipitation rates have resulted in minimal fluvial washout but the massive accumulation of dust and salts such as sulfates that often overlie chloride and nitrate horizons, which are cementing the upper meters of the soil 12,13,[20][21][22] . This regionally ubiquitous soil is located on inactive and morphologically stable alluvial deposits 23 , and has been classified into different locally termed horizons, which can vary in degree of development and thickness 21 : The upper ~ 30 cm are called chusca, which is a gypsic horizon, i.e., a non-indurated gypsum-rich horizon, that consists of a few centimeter thick layer of loose sediment containing palm-sized gypsum-anhydrite aggregates termed losas underlain by a vesicular horizon that is composed of powdery, and porous gypsum and anhydrite. Below lies the costra, a petrogypsic horizon, i.e., indurated gypsum-rich horizon, consisting of sandy-to-gravelly sediment that is intensely cemented by low soluble sulfates (gypsum, anhydrite and bassanite) and can extend up to a thickness of 2 m. Underneath the costra horizon is the caliche, a petrosalic horizon, i.e., a horizon indurated by salts more soluble than gypsum 24 , which are in this case mainly nitrates and chlorides, extending up to a thickness of 5 m.
PNs occurring on these soils in the Atacama Desert ( Fig. 1) exhibit ~ 1 m deep and salt-poor sand wedges outlining salt-rich polygons of a uniform flat-center morphology 25 . They exhibit an orthogonal to hexagonal pattern geometry, and elongated polygons have either one or two dominant orientations, preferably parallel or perpendicular to the slope. However, the details on the formation of PNs remain under debate, such as the driving forces behind the crack formation. The proposed processes range from thermal contraction, where cementation by salts results in cohesive soils and makes them susceptible to thermal stress 25 and/or salt dehydration 12,14 , both being cyclic processes. The details on the importance of rain events for PN formation remain unclear, although, its involvement is certain as rainwater infiltration, which is insufficient to fully wash out the salts, creates the vertical salt sequence of these indurated soils 12,22 . To evaluate the use of PNs as a proxy for climatological and soil conditions, we investigated a 200 m long PN that is exceptional in that it displays multiple and novel morphological and geochemical variations.

Results
The morphological and geometrical characterization of the investigated PN and its surrounding was based on two drone-based digital elevation models and orthophotos here referred to as the overview model (Fig. 1A) and close-up model (Fig. 1B). The sedimentary and geochemical characterization of three surface transects and two depths profiles was done through field observations and sample analysis of their soluble content, grain size distribution, and mineralogical composition using X-ray diffraction (Fig. 2). The investigated PN is located within an elongated depression of an alluvial fan, however, only the drone-based topography reconstruction and channel modeling show clearly that the PN lies within an inactive channel (Fig. 1A). The surface of the PN is littered with gravel and boulders, while sand dominates the upper 10 cm (Fig. 2H). The polygons share a similar clastic mineralogy dominated by quartz, feldspars, and minor amounts of amphiboles ( Supplementary Fig. S3). All polygons show a relatively similar size with a mean diameter of 3.9 ± 1.7 m and exhibit connected sand wedges forming a continuous network with a primarily hexagonal pattern geometry with two main polygon orientations to the NS and EW ( Supplementary Fig. S1), consistent with a uniform and simultaneous formation for the entire PN.
As described above, the PN exhibits similarities regarding geometry, orientation, grain size distribution, and mineralogy of the non-soluble fraction (Fig. 2H, Supplementary Fig. S1, Supplementary Fig. S3). However, we distinguish three polygon types (P1, P2, P3) within this network based on variations in surface morphology and mineralogy of the cementing salts (Figs. 1, 2 and 3): The P1 polygons located upslope of the network, show a flatcenter morphology and are outlined by dark varnished and broad sand wedges (Fig. 2), analogous to the common polygons which are visually recognizable on 28% of the reconstructed surrounding terrain of 0.73 km 2 (Fig. 1). Their surface exhibits salt-poor sediment and embedded losas (palm-sized gypsum and anhydrite aggregates), which is typical for the upper part of the chusca horizon (gypsic horizon) (Fig. 2). The P2 polygons located up to mid-slope show a high-center morphology with depressed and broad sand wedges (Fig. 2). In contrast to the P1 surface, the surface of the elevated P2 polygons lacks the loose sediment cover, is more indurated, and exhibits a high average content of soluble salts of 36% along the polygon transect dominated by sulfates (gypsum and anhydrite), which is a soil build-up similar to the costra horizon (petrogypsic horizon). The P3 polygons located mid-to downslope show a low-center morphology with narrow incised sand wedges bound by elevated polygon shoulders (Fig. 2, Supplementary Fig. S5). The polygon is composed of a mix of salts of high solubility (halite, niter, and nitratine) and low solubility (gypsum, bassanite, and anhydrite), which is a soil composition similar to the upper part of the caliche horizon (Fig. 2). The polygon surface exhibits domed salt-sediment crusts and cavities (Fig. 3, Supplementary Fig. S5). The salt content, mainly halite, nitratine, and darapskite, increases up to 74% towards the base of the polygon profile at a depth of 60 cm (Fig. 2, Supplementary Fig. S2). The investigated sand wedge contains less salts than the polygons but shows a similar increase in salt content with depth, reaching 35% at its base at 50 cm depths.
A central observation is that the downslope sequence of the exposed polygon surfaces (P1, P2, P3) corresponds to the vertical sequence of soil horizons with chusca, costra, and caliche ( www.nature.com/scientificreports/ erosion. We interpret the exposed sulfate-cemented surface at P2 as the costra horizon (petrogypsic horizon), a horizon below the chusca. The relief at P2 with a high-center morphology and broad troughs above sand wedge is consistent with eolian erosion, whereby the less cemented sand wedges are more intensely eroded than the cemented polygon center. We interpret the nitrate-and chloride-containing surface at P3 as the caliche horizon (petrosalic horizon) which underlies the costra, exposed by additional surface erosion (Fig. 3). We estimate an eolian erosion of ~ 50 cm on P3 surfaces, based on the lack of a chusca and costra horizon, which have a thickness of 20 and 30 cm, respectively, in a soil pit in a proximity of 200 m 25 (Fig. 3). This erosional depth is supported by the observation that the sand wedge at P3 is 50 cm deep, while non-eroded sand wedges in the nearby soil pit have a depth of 100 cm. Further indication for eolian erosion is the observed decrease in sand wedge width from P1 to P3 polygons, while polygon size and geometry are similar. We interpret this decrease in sand wedge width to result from the erosion of the upper wider parts of the sand wedges exposing their narrower base. The eolian erosion being caused by the strong westerly winds in the Yungay valley 16 is supported by the preferential erosion of the windward sides (west-facing) of the sand wedge troughs (Supplementary Fig. S4) and abundant scour marks around the base of boulders ( Supplementary Fig. S5). Further evidence for the eolian erosion of a preexisting PN is the relation between slope and polygon alignment. Comparing the alignment of the polygons with the average slope direction of the surrounding area (250 × 400 m, dashed box in Fig. 1A) results in a perpendicular or parallel slope alignment ( Supplementary  Fig. S1). Such a relation is well known from periglacial polygons 2 and has also been shown for PNs in the vicinity of the study area 25 . However, this alignment is lost when comparing the polygon orientation with the slope direction of each individual polygon surface using the close-up model (Supplementary Fig. S1). This deviation is consistent with differential eolian surface erosion changing the slope direction but not polygon orientation. Besides changes in morphology and geochemistry, the erosion led to a "pattern inversion" as the common flat-center polygons appear bright and are outlined by dark sand wedges, whereas at P3 surfaces the polygon interior is darker than their narrow sand wedges (Fig. 1). This shift to a darker polygon color is probably due to the removal of the bright sulfates. The darker appearance was intensified when the hygroscopic salt crusts at www.nature.com/scientificreports/ P3 became wetted by deliquescence during periods of high air humidity ( Supplementary Fig. S5) 26 . Such cycles of minimal moisture input due to dew and fog probably resulted in the observed clast encrustation by salts and undulating and domed salt-sediment crusts ( Supplementary Fig. S5). At the same time, downward leaching by rare rain events is indicated by the lower halite content in the first 10 cm at P3, compared to depths below 10 cm (Fig. 2). These rain events may have further modulated the salt crusts because ephemeral salt efflorescence has been observed on the surface of many soils after the recent rain events 14 .

Discussion
Development and erosional history of the polygonal network. The eolian erosion of the here investigated PN resulted in the exposure of subsurface horizons, which allows for the possibility to acquire additional information particularly through remote sensing if no in situ field observation is possible. However, only using remote sensing data for interpretation without in situ field observations and subsequent sample analysis, can lead to misinterpreting the lateral variations in morphology and composition to result from fluvial erosion or different formation processes. Based on our findings in the context of the established knowledge on the past climate and soil formation processes in the Atacama Desert, we propose the following developmental history for the here studied PN. The alluvial fan is the host material of the PN and at most of Miocene age 27 . The channel activity in the study area probably ceased in the Late Pliocene to Early Pleistocene and was followed by reduced precipitation and massive salt accumulation 13 . Consequently, we assume that PN formation, continuing until today, began after the Late Pliocene (~ 2.6 Ma) and the incision of the channel but probably required first a sufficient accumulation and cementation by salts for initiation 12,13 . These salts, indurating the soil are a prerequisite for polygon development, presumably due to dehydration of sulfates 12,13 and/or due to thermal contraction in salt-cemented ground 25 . Furthermore, the presence of highly soluble salts at shallow depth indicates an incomplete washout as a result of a predominantly hyper-arid climate punctuated by rare rain events, as suggested before [12][13][14]22 . Hence, the relationship between the high soluble salt content and hyper-arid conditions on the one hand and the presence of PNs that developed under these conditions, on the other hand, allows for the use of PNs as a proxy for saline soils (pedological condition) and predominantly hyper-arid conditions. Advantageously, PNs can be frequently detected with remote sensing using e.g., satellite imagery, which makes them a suitable environmental proxy at the regional scale. However, their surficial detection can be hindered by sediment covering, e.g., by boulder fields or eolian deposits controlled by local wind pattern 22 . Thus, the inability to observe PNs neither necessarily implies that a polygonal ground is lacking nor that saline soils and hyper-arid conditions are absent.
Insights from eroded polygonal networks. The differential erosion of PNs can provide further insights into surface and subsurface characteristics: (1) The exposure of the subsurface geometry of wedges can be used to deduce the stress regime, the cracking depth and cementation depth. Yet, the quantification of the sand wedge geometry based on aerial imagery is not always possible in the visible spectrum and could be aided by e.g., spectral imaging highlighting material differences. (2) Differences in polygon morphology can indicate variations in sediment induration and therefore resistance to eolian erosion, e.g., the development of a high-center morphology at P2 surfaces is indicative of less cemented sand wedges compared to the elevated polygon center. Further, the relief development of elevated polygon shoulders, e.g., at P3 surfaces may point to salt-related ground deformation, for example, due to the thermal expansion of the now exposed highly soluble salts during ground heating, also known from periglacial polygons 28 or volumetric expansion based on the hydration of sulfates after moisture input. (3) The surficial exposure of highly soluble salts at P3 provides information on the compositional stratification of the soils, which in our case results in a pattern inversion from bright polygons outlined by dark sand wedges (P1) to dark polygons with surface crusts outlined by bright sand wedges (P3). Furthermore, the presence of these crusts can provide insights on the rates of concurrent processes continuously reshaping the soils. While on the one hand eolian erosion promotes the exposure of the soluble subsurface salts, on the other hand rain events will inevitably transport the salts back into the subsurface. Hence, we can conclude, that the rate of eolian erosion has been far more rapid than the down washing of the salts. However, the occurrence of surface crusts indicates surficial alteration that is more rapid than eolian erosion. Presumably the surface crusts have been created by minor rain events with minimal soil infiltration and fog that frequently occurs up to 90 km inland and below elevation of 1000-1300 m a.s.l. 22,29,30 . (4) The depth-related sequence of salts of different solubility can be used to infer the dominating water pathways (rain vs. groundwater), as is the case for this PN where the solubility increases with depth corresponding to a downward migration of rainwater, which is common for alluvial surfaces in this area 12,22 . Interestingly, recent unusually frequent rain events, as recorded in 2015 and 2017, have been linked to climate change 31 ; such an increase in precipitation will eventually washout the nearsurface salts and inevitably change the PNs, which thus can serve as a proxy for climate change. (5) Lastly, our study contributes to a better understanding of the dust sources and sinks in the Atacama Desert, as we show that the saline subsurface material of PNs can at least be locally remobilized through eolian erosion and introduced into the dust cycle 18 . Given this applicability, differential erosion of PNs is of particular value in remote locations where field sampling is not always possible but remote sensing can be applied. Hence, our findings can serve as a reference example and guideline for PN evaluation in the Atacama Desert and in other arid environments affected by dry erosion, such as the Gobi Desert 32 , the Dry Valleys of Antarctica 33,34 , and on Mars 5,35 . www.nature.com/scientificreports/ chloride-containing soils such as the southern Martian highlands 39,40 . Lateral changes in PN morphology and/ or composition have also been observed on Mars, e.g., in Utopia Planitia where low-center polygons occur in a topographic low and transition further upslope into high-center polygons that become flat accompanied by a widening of their sand wedge troughs 35 . Although this depression is altered by sublimation and margin collapse, the morphological transition is strikingly similar to our findings in the Atacama Desert. However, the Martian low-center polygons were interpreted to be aggrading and the high-center polygons degrading, whereas in this study these types were both degrading due to wind erosion. Based on the observations in the Atacama Desert, we think that also low-center polygons can have not only aggrading but also degrading wedges. Hence, we suggest that wind erosion, known to be a dominant modifier of the Martian surface 41 , should be considered when evaluating the PNs on Mars or elsewhere. Lastly, thermal contraction and sulfate dehydration, which are both processes proposed to play a role for PN development in the Atacama Desert could be relevant for polygon development on Mars as well. In particular, polygons in Meridiani Planum bear similarities to sulfate-containing polygons in the Atacama Desert and have been interpreted to result from wetting/drying cycles causing sulfate re-/dehydration 42 , which could have implications for their potential habitability. Additionally, sulfate 40 and/ or chloride-containing ground 39 would allow for thermal contraction cracking due to salt cementation, which would be an additional mode to the common thermal contraction in ice-cemented material 5,43 .

Conclusion
We investigated a polygonal network (PN) where eolian erosion of flat-center polygons resulted in high-center and low-center polygons and the exposure of subsurface horizons with varying geochemistry. In general, PNs develop in saline soils under hyper-arid conditions in the Atacama Desert, promoting them as a proxy for these conditions. Consequently, a shift to moister conditions would be reflected in the PNs making them a valuable marker for climate change. In particular, eroded PNs provide a window into the subsurface revealing further insights regarding ground conditions and development. Hence, PNs affected by erosion need to be considered for the assessment of polygons in hyper-arid environments, in particular on Mars, where the wind is a dominant surface modifier and PNs frequently occur on saline ground. Future polygon monitoring and research, investigating their cracking mechanisms, formation rates, and occurrence in the Atacama Desert are greatly needed, to better understand patterned ground formation in non-polar areas and the entirety of proxy information they can provide.

Methods
The sedimentological, geochemical, and morphological properties of the PN (− 24.07796, − 69.99339) were determined as follows. Three 5 m long soil transects of each polygon type (P1: 12 samples, P2: 11 samples, P3: 12 samples), stretching across a single polygon and its neighboring polygon shoulders were sampled in a depth of 0-10 cm for grain size distribution, X-ray diffraction (XRD) and salt content. Additionally, two excavated profiles at P3 along the polygon (6 samples) to a depth of 60 cm, and the sand wedge (5 samples) to a depth of 50 cm were sampled in ~ 10 cm increments. Drone flights, 3D-spatial data generation, geochemical, sedimentological, and morphological analyses were conducted using similar methods as described by Sager et al. 25 .
Agisoft Metashape was used for photogrammetric reconstructions to generate digital elevation models (DEM) and orthophotos 44 . Two models were generated, here referred to as the "overview model", which displays the PN and the surrounding topography of the alluvial fan, while the high-resolution "close-up model" displays solely the PN. Polygon mapping, analysis, and map generation were conducted with QGIS 45 . Orientational data was plotted using GeoRose 46 . For grain size analysis, samples were leached with de-ionized water and wet sieved. The salt content was calculated based on the weight difference of samples before and after leaching. For XRD analysis, dry-grinded samples and a Bruker D2 Phaser benchtop diffractometer were used, and semi-quantitative (SQ) analysis was carried out with the "Diffrac.eva" software and the "Powder Diffraction File Minerals 2019″ (database of the international centre for diffraction data) to determine the mineral content. The accuracy of the determination of the total soluble content based on weight differences after leaching is higher compared to the salt content based on SQ-XRD. Therefore, we calibrated the relative abundances of the identified mineral assemblages to the total salt content determined by sample leaching. Detailed methods are provided in the Appendix (Supplementary File 1).